1. Field of the Invention
The present invention concerns a method in which at least two 3D image data items of a patient are geometrically correctly associated with one another.
2. Description of the Prior Art
In the operative treatment of fractures of long hollow bones, care must be taken that the corresponding extremity axes of the patient are established in the original form. For example, in the case of a femur fracture the leg axis (avoiding varus or valgus alignment), the leg rotation and the leg length must be set correctly.
It is known to visually check the leg axis interoperatively with an auxiliary means. The cable of an electrocauterization tool is often used for this purpose. It is also known to measure the leg length in comparison to the healthy leg of the patient and to visually compare the leg rotation with the healthy leg.
A 3D x-ray method and apparatus (for example a 3D x-ray C-arm) are often employed intraoperatively. Such a method or apparatus generates a 3D image data set of the patient in the form of a reconstructed spatial region (shortened in the following to “3D image data”). For acquisition, the apparatus is fixed in a 3D acquisition position. For example, for the aforementioned C-arm a basic support is fixed at a specific spatial position in the operating room (OP). The x-ray apparatus is moved between different positions in this invariant basic alignment. For example, in the case of the aforementioned C-arm the C-arm of the apparatus is moved orbitally between different positions with an immobile basic support. A set of 2D x-ray images or 2D projection images is thus acquired from which the 3D image data are then reconstructed.
Such 3D (normally mobile) x-ray apparatuses that can be used intraoperatively most often have a relatively small reconstruction volume for the 3D image data, for example a cube with edges of only 15 cm in length. The volume is directly coupled to the size of the x-ray detector that is used. A long hollow bone is multiple times longer, so that multiple 3D image data of the patient must be acquired in order to image the entire bone. Newer methods enable the geometrically correct composition of individual 3D image data into a complete 3D image from which the leg axis and the leg length can be derived.
“Geometrically correct” means that, with regard to the imaged patient, the non-contiguous 3D image data are spatially accurately associated with one another relative to form an imaginary complete image (thus like spatial sections that are spatially fixed relative to one another). They thus depict the real relationships to the patient with spatial accuracy through their mutual attitude and orientation. The partial volumes can be contiguous (i.e. overlap), but need not be, in order to image a larger patient volume as a whole.
There are two approaches for combination (“stitching”) of the 3D image data. In the first approach, multiple 3D image data are acquired with overlapping of respective images or spatial regions that are sufficient in pairs in order to generate contiguous 3D image data from these. Image fusion algorithms are used for this purpose.
In the second approach, the individual 3D image data are acquired so as to not overlap; for example, first 3D image data of the hip, second of the knee and third of the ankle of the same patient are acquired. Due to the lack of overlap, however, here a uniform reference system or coordinate system must be achieved for all (partial) data sets (thus 3D image data) in order to geometrically correctly arrange the individual 3D image data or bring them into congruence. For example, here the respective position determination of the imaging system using a commercially available tracking (i.e. position detection) system would be conceivable. However, this requires additional hardware and software cost and therefore is hard for the user to accept.
There are numerous possibilities for realization for the stitching or other correct spatial association methods in the 2D field. For example, a rigid marker plate with x-ray-visible markers (location codes) for the spatial association of 2D x-ray images is known from the article “Image Fusion for Intraoperative Control of Axis in Long Bone Fracture Treatment”, Peter Messmer et al., European Journal of Trauma 2006, No. 6, P. 555-561, Urban & Vogel. The marker plate is placed beneath the patient so that parts of this are visible in multiple partial 2D x-ray images (also not overlapping). The marker plate enables a geometrically correct association of the 2D individual images with one another since the x-ray-visible location codes of the marker plate are visible in the x-ray image and allow the association.
The marker plate must be placed below the patient before his positioning; a later placement is therefore no longer possible. The marker plate must be positioned and also cleaned in the operating room. Under the circumstances, the marker plate disruptively presses through the patient bed.
This approach fails for intraoperative 3D imaging with the aforementioned small reconstruction volume since the distances between marker plate and a location of interest in the patient that is to be imaged are often too large for these to be recorded in individual 3D image data.